Most classified messages have been sent encoded using a variety of cryptographic coding techniques. These techniques, while usually offering high levels of security, tend to be costly, require elaborate equipments and must have appropriately qualified personnel to operate them effectively. Over the years a variety of devices and methods have been tried to reduce the inherent drawbacks attendant the transmission of classified information among which would be the relieving of the expensive life-cycle cost and complexity of cryptography for certain levels of message classification.
Recent innovations in providing for secure transmissions have relied upon optical fibers and associated equipments. A fiber disclosed in U.S. Pat. No. 4,000,416 provides for a multipath optical communication system within a single fiber. Information is transmitted in an inner core and an outer core transmit a security signal, any interruption thereof alerts to an intrusion. The fiberoptic communication system of U.S. Pat. No. 4,174,149 sends information and an alarm signal within a frangible fiber that provides an indication of an intrusion. The system of U.S. Pat. No. 4,207,561 injects an intelligence modulated beam into an optical waveguide via a beamsplitter and delays a replica portion, a time equivalent to the transmission time through a waveguide and later correlates it to determine an indication of interception of the information through the waveguide. The secure optical communication system of U.S. Pat. No. 4,211,468 launches information along with masking signals at different angles into an optical waveguide to assure a secure communication capability. The secure system of U.S. Pat. No. 4,217,488 has optical fibers reaching between a transmitter and receiver carrying information as well as guard signals each discreetly sent on separate fibers reaching between two terminals.
Thus there is a continuing need in the state-of-the-art for a secure communication system that advantageously includes the benefits of single-mode fiber technology for providing a low-cost, highly secure communication system not requiring unduly special equipments or operator training for its successful operation.